1. Field of the Invention
This invention relates to non-aqueous electrolytic secondary cells and more particularly, to a method for preparing an active substance for positive electrode for use in the cells of the type mentioned above.
2. Description of the Prior Art
Spinel type LiMn.sub.2 O.sub.4 is known as a positive electrode active substance for non-aqueous electrolytic secondary cells which make use of lithium or lithium alloys as a negative electrode (Material Research Bulletin, 18, (1983) 461-472). For the preparation of LiMn.sub.2 O.sub.4, there is known a solid phase process wherein various types of lithium salts and manganese salts are mechanically mixed and fired. For instance, Japanese Laid-open Patent Application No. 2-139860 proposes a solid phase process wherein Li.sub.2 O, LiOH.H.sub.2 O or the like and .gamma.MnO.sub.2 are provided as starting materials, mixed together and heated.
Where the active substance is prepared by firing for 4 to 5 hours according to the known solid phase process, the firing temperature is set at a level as low as not higher than 500.degree. C. By this, the resultant active substance is less degraded but has a reduced capacity. On the contrary, when the firing temperature exceeds 700.degree. C. or over, the resultant active substance has an increased capacity but undergoes a substantial degree of degradation, particularly of initial degradation. Up to now, it has never been attained to obtain an active substance for positive electrode which has a great capacity and a reduced degree of degradation.
The solid phase process makes use of a reaction which depends on diffusion and thus, takes a very long time before the active substance acquires a uniform composition therethroughout. This is considered for the reason why with the firing, within such a short time as set forth above, the compositional variation becomes great resulting in the formation of an active substance which has a reduced capacity and a great degree of initial degradation.